The invention relates to a compact electric motor, and more particularly to a structure of a motor housing with a built-in stator.
Conventionally, small size electric motors such as stepping motor have a housing for accommodating a built-in stator made of, e.g., an aluminum alloy. A conventional electric motor of this type will be described with reference to FIG. 2.
FIG. 2 is a sectional view showing a control valve device using the conventional small size electric motor. In FIG. 2, reference numeral 1 designates a valve body; and 2, a drive unit which is secured to the valve body 1. These components 1 and 2 constitute the control valve device. The valve body 1 is provided with a path la for flowing a control fluid and a valve seat 1b confronting a valve piece (described later).
The drive unit 2 includes a stepping motor 4 for rotating a rotor shaft 3, a holder 5 for converting the rotary motion of the rotor shaft 3 into a linear motion, and the valve piece 6 secured to the front end of the holder 5.
The rotor shaft 3 of the stepping motor 4 is rotatably supported by a bearing 8 and a bearing 10. The bearing 8 is arranged on an inner peripheral portion of a housing 7 that is secured to the valve body 1, and the bearing 10 is arranged on an inner peripheral portion of a cover 9 that covers the rear end of the housing 7. The housing 7 is made of metal such as an aluminum alloy. A multipolar magnet 11 is secured around the outer periphery of the rotor shaft 3 by a mold 12 so as to be concentric with the rotor shaft 3. A stator 13 is arranged on the outer peripheral side of the magnet 11. Reference numeral 14 designates a coil of the stator 13; 15, a mold for protecting both the coil 14 and a coil terminal 16; 17, an external connection terminal connected to the coil terminal 16; 18, a packing interposed between the housing 7 and the cover 9; and 19, a spring for preventing the rotor shaft 3 from backlashing in the axial direction, the spring being arranged between the cover 9 and the bearing 10 so as to act resiliently therebetween.
The holder 5, which is substantially cylindrical, is made of a synthetic resin and is supported so as to be inserted into the rotor shaft 3 while passing a portion of the rotor shaft 3 therethrough, the portion projecting from the housing 7. The valve piece 6 is fitted into and secured to an opening on the front end side of the holder 5. The valve piece 6 is also formed of the synthetic resin unitarily. The base portion on the side of the projecting portion of the rotor shaft 3 is provided with a male screw, while the inner peripheral portion of the holder 5 is provided with a female screw that ca be screwed into the male screw. A guide 20 serves to eliminate the rotation of the holder 5 caused by the rotary motion of the rotor shaft 3. The guide 20 is secured to the housing 7. More specifically, when the rotor shaft 3 is rotated by the stepping motor 4, the holder 5 gets screwed into the rotor shaft 3, while held from rotating by the guide 20. As a result, the rotary motion of the holder 5 is converted into a linear motion, so that the holder 5 travels along the length of the rotor shaft 3. A spring 21 that serves to prevent the holder 5 from moving back and forth slightly due to backlash of the screwed portion is interposed between the holder 5 and the guide 20 so that the holder 5 can be urged to the front side (toward the valve seat 1b).
A stopper 22 that serves to regulate the maximum advance position of the holder 5 is held by a stopper ring 23 at a front end of the rotor shaft 3. Reference numeral 24 designates a stopper for regulating the maximum retreat position of the holder 5.
In the control valve device thus constructed, the holder 5 that is screwed into the rotor shaft 3 travels frontward and backward by rotation of the stepping motor 4. This causes the valve piece 6 to move back and forth to change the distance between the valve seat 1b and itself, i.e., the area of the fluid path, thereby controlling the flow rate of the fluid flowing through the path la. The maximum advance position of the valve piece 6 is regulated by the abutment of the holder 5 against the stopper 22, while the maximum retreat position thereof is regulated by the abutment of the holder 5 against the stopper 24.
Accordingly, the thus constructed conventional control valve device is disadvantageously heavy in weight. This is because the housing 7 of the stepping motor 4 is made of the aluminum alloy. To overcome the above inconvenience, the housing 7 may be formed of a comparatively lightweight material such as a synthetic resin. However, a change in the material to a synthetic resin creates the problem of radiating heat, generated by energizing the coil 14, outside the device because the heat conductivity of the synthetic resin is low compared with that of a metal.